To prevent superimposition of external electromagnetic waves as noise on input and output signals to and from electronic components (components that generate heat while electronic devices are being operated) as well as superimposition of electromagnetic waves generated from the electronic components themselves as noise on other signals, it has conventionally been proposed to shield electromagnetic waves entering and leaving the electronic components. Electromagnetic shielding cases that are configured to cover from above one or multiple electronic components mounted on a printed circuit board with a metal case are known.
If the above structure is used, however, the electronic components are then hermetically closed and thus have problems such as that the electronic components are likely to undergo an increase in temperature and degradation, or are less likely to exert their properties, compared to other components, because these electronic components are covered by air which is a poor conductor of heat, although electromagnetic shielding properties are not adversely affected. In particular, since recent electronic components generate increased heat densities, countermeasures against heat are essential.
To provide a countermeasure against heat in such a system, Patent Literatures 1 and 2 disclose techniques in which a resin is filled into a hermetically closed space formed by a sheet metal case for electromagnetic shielding to release heat generated from electronic components mounted in the case to the outer surface of the case. However, since the thermally conductive resins disclosed are silicone resins, there is a concern regarding contact failures in electronic components due to volatilization of low molecular siloxane components or cyclic siloxane components.
Moreover, general materials used as countermeasures against heat include thermally conductive grease disclosed in Patent Literature 3 and a thermally conductive sheet disclosed in Patent Literature 4. Yet, unfortunately, the former material may leak from the system because it does not cure by nature; the latter material is not suited to fine bumps on electronic components. Thus, these materials are inadequate as countermeasures against heat from electronic components in such an electromagnetic shielding case described above.
In addition, recent personal digital assistants (e.g. smartphones and tablets) that are gaining worldwide popularity include electronic components whose operating speeds are rapidly increasing, which accompanies an increasingly large amount of heat generation per unit time. However, personal digital assistants such as smartphones and tablets cannot be provided with sufficient space for heat dissipation. Without efficient heat dissipation, their electronic components can have the problem that they will readily undergo an increase in temperature and degradation, for example. Thus, currently, there is a demand for electromagnetic shielding cases having significantly high heat dissipation efficiency as compared to conventional products.
Patent Literature 5 discloses a thermally conductive material formed from a curable acrylic resin containing a crosslinkable functional group and a thermally conductive filler. This thermally conductive material, which has not only high thermal conductivity but also fluidity before curing, can exhibit good adhesion to bumpy objects, unlike sheet-like or gel-like thermally conductive materials, thus being capable of suppressing an increase in contact thermal resistance resulting from falling off during use, air gaps, or the like. In addition, since it cures at room temperature, leakage of the thermally conductive material from the system over time, which is a problem associated with grease-like thermally conductive materials, will not occur, and volatilization of low molecular siloxane components or cyclic siloxane components, which is a problem associated with silicone-based thermally conductive materials and causes contact failures in heat-generating electronic components, cannot occur. Thus, the thermally conductive material is excellent in long-term stability.
However, thermally conductive materials are required to have handleability and workability at work sites and maintenance sites, and in particular, they need to be able to be easily peeled off from a heat-generating element or a heat-dissipating element in a process of removing the applied layer of thermally conductive material at the time of repair, inspection, or replacement of components (repairing step) and, even if the layer of thermally conductive material partially remains, the residue needs to be usable by performing a jointing process, without deterioration in properties.
In regard to such peelability of the thermally conductive material layer, for example, Patent Literature 6 discloses a curable silicone resin having improved peelability. However, since a silicone composition is used, the above-described problem of volatilization of low molecular siloxane components still exists. Furthermore, Patent Literature 7 discloses a technique relating to a curable polyisobutylene resin whose backbone skeleton consists of polyisobutylene.